Tracking the 2007–2023 magma-driven unrest at Campi Flegrei caldera (Italy)

Assessing the eruption probability during volcanic unrest requires understanding magma distribution and migration within the shallow plumbing system. Critical parameters include the depth, size, and magma transfer between melt pockets. This is particularly crucial for long-lasting unrest episodes near densely populated areas, necessitating close monitoring of the evolving shallow plumbing system. Surface deformation, often monitored through geodetic data, is a key indicator of pressure variations in underlying sources. However, interpreting these sources (magmatic vs. hydrothermal) can be challenging, especially at intermediate depths (3-4 km). The Campi Flegrei (CF) caldera, Italy, exemplifies this challenge, with its main unrest source inferred at intermediate depths, leading to conflicting hypotheses regarding its nature. This study aims to better constrain the nature of the CF shallow source by integrating geodetic data, numerical modeling, and petrological calculations. CF has experienced several unrest episodes since its last eruption in 1538 CE (1950-1952, 1969-1972, and 1982-1984), each marked by uplift in the Pozzuoli area. Since 2005, the caldera has been slowly uplifting at increasing rates, reaching a cumulative uplift of 119 ± 0.3 cm at Pozzuoli by December 2023. This reawakening was accompanied by increased seismicity, particularly since 2014, and changes in the Solfatara-Pisciarelli fumarolic field's degassing rates and geochemistry. Petrological and geochemical analyses suggest that a significant portion of the emitted gas originates from a deep magmatic system located at depths ≥8 km, implying a high-melt percentage layer at about 7.5 km. Repeated unrest episodes may have altered the crust's thermal and rheological properties at CF, creating conditions potentially favorable for an eruption. This work aims to precisely define the ongoing unrest at CF in terms of the location, size evolution, and nature of the involved sources.

Previous studies of Campi Flegrei's unrest have employed various techniques and yielded differing conclusions on the depth and nature of the deformation sources. Some studies using continuous GPS data suggested a magmatic source at depths greater than 3 km, while others indicated the involvement of hydrothermal processes. The interplay between the shallow and deep sources and the transfer of fluids or magma between them has not been clearly established. While the deeper source (≥8 km) is generally considered to be magmatic, the nature of the shallower source remains controversial. Different modeling approaches—involving homogeneous or heterogeneous media assumptions—led to variations in the modeled depths of the main source, ranging from ~2.5 to ~5 km. Although previous works suspected magma transfer from the deeper reservoir to a shallower source, particularly during the 1982–84 unrest, this was not fully constrained for all periods. The presence of a two-source plumbing system, with a shallower magmatic body at 4.5–5 km depth since 2019, has been supported by recent 4D tomography. The ambiguity surrounding the mechanisms driving the ongoing unrest—magma intrusion, fluid migration, or hydrothermal system perturbations—highlights the need for a more comprehensive understanding to evaluate eruption potential.

This study integrates multiple datasets and modeling techniques to analyze the Campi Flegrei unrest from 2007 to 2023. Geodetic data comprised inland and offshore GNSS networks and SAR images from ENVISAT (2007–2011) and COSMO-SkyMed (2011–2023) satellite missions. These data were analyzed to define seven time intervals reflecting the non-linear uplift trend. Finite Element Method (FEM) modeling, incorporating the elastic structure of the local crust inferred from active and passive seismic tomographies, was used to model the deformation source. The model included a shallower source (shape not a priori defined) and a deeper source fixed at 8 km depth. A Bayesian inference approach was applied to the geodetic velocities during each time window to retrieve optimal source parameters (location, volume changes, and overpressure). Petrological simulations of mass balance were then performed to determine the likely nature of the sources and the mass transfer between them. These simulations explored several scenarios, including: 1) volatile transfer only, 2) volatile and magma transfer, 3) volatile exsolution due to cooling and crystallization, 4) magma ascent, degassing, and fluid release, and 5) direct magma transfer from the deeper to shallower source. Equivalent strain calculations (J2-norm of the strain tensor) from the FEM models were used to assess the correlation between the stress and strain regime induced by the unrest and the distribution of seismicity at different crustal levels (700 m and 1400 m).

The analysis revealed a two-source plumbing system at Campi Flegrei: a shallower source exhibiting progressive shallowing and widening (from 5.9 km to 3.9 km depth between 2007 and 2023) and a deeper source at 8 km depth undergoing limited but persistent deflation. The shallower source's volume increased from approximately 0.1 km³ in 2007 to 0.16 km³ in 2023. The deeper source showed a deflation of about 5 million m³ over the 16-year period. The seismicity predominantly concentrated above the shallower source and to its northeast. FEM modelling successfully reproduced the observed surface deformation, with residuals demonstrating that most of the deformation is captured by the model. Equivalent strain calculations showed an annular pattern with the highest strain concentrations (indicating areas most susceptible to failure) located near the Solfatara area, consistent with the observed seismicity pattern. Petrological calculations indicated that the most plausible scenarios to explain the observed geodetic data involve magma ascent from the deeper reservoir to shallower levels, either releasing excess fluids at intermediate depths or directly contributing to the inflation of the shallower source. Calculations suggest that the rise of 0.06 to 0.22 km³ of magma from depths ≥ 8 km is required to explain the observed shallower source inflation, compensated for by magma replenishment from even greater depths. The precise volume of ascending magma cannot be fully constrained, as the volume increase could result from a larger volume of magma releasing fluids at shallower depths or a smaller volume ascending directly to shallower levels.

The findings demonstrate a clear connection between magma ascent from depth and the surface deformation observed at Campi Flegrei. The integrated geodetic and petrological approach provides stronger evidence for magmatic processes driving the ongoing unrest compared to previous studies. The shallower source's shallowing and widening over time indicates a complex interplay of pressure changes, magma migration, and potentially structural controls. The correlation between the modeled strain field and observed seismicity underscores the importance of considering the 3D elastic structure and non-axisymmetric source shapes in interpreting unrest signals. The uncertainty in pinpointing the precise volume of ascending magma highlights the need for continued research focusing on improved model constraints and the inclusion of factors like viscoelasticity and time-dependent rheologies. The results emphasize the potential for magma movement to directly contribute to the unrest and increase the likelihood of eruption, requiring close monitoring of the caldera's activity.

This study provides a comprehensive analysis of the Campi Flegrei caldera's unrest from 2007 to 2023, demonstrating a clear link between deep magma ascent and surface deformation. The integrated geodetic and petrological modeling approach significantly improves constraints on magma dynamics during volcanic unrest. Future research should focus on refining the models by incorporating more detailed information about the crustal structure, viscoelastic rheology, and time-dependent processes, and further integrating geochemical and geophysical data to provide a more complete picture of the magmatic system's evolution and eruptive potential.

The study's limitations include the simplifying assumptions made in the FEM modeling, such as the idealized source shapes and the neglect of potential mechanical interactions between the shallower and deeper sources. The petrological calculations provide first-order estimates and rely on certain assumptions about magma composition and volatile content. Furthermore, the purely elastic model may not fully capture the complex time-dependent rheological behavior of the caldera's crust. Finally, although the data were updated to October 2023, more recent data could further refine the findings and might reveal changes not captured in this timeframe.